1. Field of the Invention
The present invention relates to a feature quantity calculation device and a method thereof for calculating a 3-D (three-dimensional) object's feature quantity capable of being used for recognition of 3-D objects and retrieval of similar 3-D objects from 3-D object data contained in the 3-D object.
2. Description of the Related Art
In recent years, data formats for expressing 3-D objects are widely used for representing CAD (Computer-Aided Design) data, 3-D object data for merchandise, archaeology, and works of art, digital archive of data, etc. These types of data are increasing steadily. There is an increasing need for efficiently managing stored 3-D object data and effectively retrieving 3-D object data requested by users.
Various technologies are proposed to satisfy these demands. In a technology of retrieving “similar objects”, there is proposed a method of calculating a feature quantity included in an object and performing a retrieval according to the calculated feature quantity.
For example, the following technology is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 6-215105. The 3-D geometrical information mainly comprises vertex coordinates as geometrical information about vertexes of a formed polygon and topological information (connection information) about the vertexes. As additional information, there are provided normal vector information at the vertexes, color information about the vertexes, etc. A user uses the above-mentioned predetermined pieces of information for retrieving similar objects.
In Jpn. Pat. Appln.KOKAI Publication No. 2000-222428, the following technology is disclosed. When a user retrieves similar objects, a retrieval object is simplified. A feature quantity for expressing shape is extracted from the simplified retrieval object. A similar object is retrieved by comparing the feature quantity for the above-mentioned retrieval object with that for an object to be retrieved.
According to the method, feature quantity examples include an object's centroid, distribution of distances from a centroid to each face constituting an object, an average value of angles formed by normal lines for adjacent faces constituting an object, etc.
Further, the following technology is disclosed in “A Content-based Search Engine for VRML Databases” (Proceedings, IEEE Computer Society Conference on Computer Vision and Pattern Recognition, pp. 541-546, 1998). A similar object is retrieved by calculating the feature quantity for a target 3-D object written in the VRML (Virtual Reality Modeling Language).
The example also uses the feature quantity to represent a 3-D object's shape. The feature quantity is calculated by statistically processing the standard deviation of distances from an object's centroid to each vertex or the product of an area of polygon patches by a value equivalent to a distance between the polygon patch's vertex and centroid. The calculated feature quantity is used to retrieve 3-D objects.
The above-mentioned prior art finds the feature of a 3-D object from structural information (position information, connection information about vertexes, or polygon patch) about a wire frame with respect to the shape feature.
When the above-mentioned vertex position information is used, for example, an array of vertexes has no regularity or isotropy. When the above-mentioned vertex connection information is used, no regularity is defined for a method of generating the connection relationship. Accordingly, an object comprises polygon patches with different size.
It is difficult to systematically calculate the feature quantity for all object data. There is a limited number of feature quantities that can be calculated based on the position information and the connection information about vertexes, making it difficult to define diverse feature quantities. If a user retrieves an intended object just by using a few feature quantities, accurate and efficient retrieval is difficult.
In many cases, a user retrieves similarly shaped objects with respect to similarity of outside shapes (wire frames). Structural information about a wire frame contains information about the structural feature inside the object. The information about the structural feature inside the object is unnecessary for the user to retrieve similar objects. When the user retrieves similar objects, the feature quantity is calculated by using data containing the unnecessary information, degrading the retrieval accuracy and retrieval speed.